Sensor Unit for a Vehicle

ABSTRACT

The disclosure relates to a sensor arrangement for a vehicle having at least one rotation-speed detection apparatus, which continuously detects a state variable and outputs said state variable to an evaluation and control unit which receives and evaluates signals which are output by a rotation-speed detection apparatus, wherein the evaluation and control unit carries out a first evaluation process with the detected state variable and ascertains a rotation speed of at least one vehicle wheel, the rim of said vehicle wheel being fastened to a corresponding wheel hub by means of wheel fastening means. According to the disclosure, the evaluation and control unit carries out a second evaluation process and identifies and monitors, on the basis of the continuously detected state variable, mechanical play between the vehicle wheel and the corresponding wheel hub in order to detect detached wheel fastening means.

PRIOR ART

The invention is based on a sensor unit for a vehicle in accordance withthe generic type of the independent claim 1.

Generally, wheel bolts are tightened by way of example while changingtires in a workshop and after an initial period of use (typically 80 kmis recommended) it is necessary for the driver to further tighten saidwheel bolts. This inspection is however often omitted by the driver.Should the wheel bolts become detached whilst driving, this can lead tosignificant accidents and great personal injury and material damage.Hereinunder, the term “wheel bolt” is used as a fastening element of thewheel rim so that embodiments of the present invention can likewise beused if the fastening function is provided by means of a wheel stud andthe associated wheel nut.

Sensors for detecting a wheel rotational speed and/or for detecting atire pressure in the region of the vehicle wheels are known from theprior art. Systems that identify loosened wheel bolts in the normaldriving operation of the vehicle are not known.

JP 2008157663 A discloses a device that determines the wheel rotationalspeed by way of an electromagnetic rotational speed sensor. In addition,a vibration sensor for detecting vibrations and a temperature sensor fordetecting temperature are used in the corresponding wheel bearing.Possible abnormalities in the rotational speed sensor or in a sensorcable or in the wheel bearing are identified by way of a further device.

DISCLOSURE OF THE INVENTION

In contrast, the sensor arrangement in accordance with the invention fora vehicle and having the features of the independent claim 1 has theadvantage that it is possible based upon the measured data of arotational speed sensor to identify in the normal driving operation ofthe vehicle that wheel bolts have become loose. This means that loosenedwheel fastening means can be continuously identified at a vehicle wheelin the normal driving operation and without additional sensors ifdevices for detecting the rotational speed of the vehicle wheel arealready installed. Embodiments of the sensor arrangement in accordancewith the invention for a vehicle are preferably installed so as todetermine the wheel rotational speed and so as to monitor the wheelfastening arrangement so that loosened wheel fastening means can beidentified at a vehicle wheel or at multiple vehicle wheels in anadvantageous manner and a corresponding error state can be output to anddisplayed on a corresponding display unit. The rotational speeddetecting device can be embodied by way of example as a magnetic oroptical rotational speed detecting device.

The fundamental advantage of the invention is that it is possible tocontinuously monitor the wheel fastening means in the normal drivingoperation and said monitoring process is possible without additionalexternal sensors. ABS sensors (ABS: Antilock Braking System) that areembodied as rotational speed detecting devices are already used in thevehicle and are widespread. It is possible at an early stage to detect aloosening of the wheel bolt and to output a corresponding warning signalin a timely manner by means of continuously processing signals andevaluating the available measuring results.

The rotational speed detecting devices that are required in order to usethe ABS functionality in the motor vehicle, said rotational speeddetecting devices generally being attached to each vehicle wheel,measure the individual rotational speed of each vehicle wheel by way ofexample by way of a magnetic multipole disc, which comprises apredetermined number of pole regions, and a magnetic field sensor. Themagnetic multipole disc is preferably installed in the wheel on thewheel bearing and is fixedly connected to the wheel. Alternatively,toothed discs or perforated discs can also be installed. The magneticfield sensor is fixedly attached on the vehicle on the other side of therotational axis, said magnetic field sensor can be embodied by way ofexample as a Hall sensor or GMR sensor. A period of time is measured byway of the magnetic field sensor, said period of time being necessaryfor the multipole disc to further rotate about a predetermined angularposition. The measurement of the movement of the multipole disc ispreferably performed by way of a measurement of the change in themagnetic field.

Embodiments of the present invention provide a sensor arrangement for avehicle having at least one rotational speed detecting device and saidsensor arrangement continuously detects at least one state variable andoutputs said state variable to at least one evaluating and control unitthat receives and evaluates signals that are output by at least onerotational speed detecting device. The at least one evaluating andcontrol unit performs a first evaluating process using the at least onestate variable that is detected and determines a rotational speed of atleast one vehicle wheel whose wheel rim is fastened to a correspondingwheel hub by way of wheel fastening means. In accordance with theinvention, the at least one evaluating and control unit performs asecond evaluating process and based upon the continuously detected atleast one state variable S identifies and monitors a mechanical playbetween the at least one vehicle wheel and the corresponding wheel hubso as to detect loosened wheel fastening means.

An evaluating and control unit can be provided for each vehicle wheeland said evaluating and control unit forms an assembly with acorresponding sensor. Alternatively, a common evaluating and controlunit can receive and evaluate the state variables that are detected byway of a corresponding sensor for each wheel. This renders it possiblefor loosened wheel fastening means to be evaluated and identified from acentral point.

Advantageous improvements of the sensor arrangement for a vehicle thatare disclosed in the independent claim 1 are possible as a result of themeasures and further developments that are embodied in the dependentclaims.

It is particularly advantageous that each rotational speed detectingdevice comprises an encoder disc having a predetermined number ofencoding regions and an allocated sensor. Each encoding region generatesin the allocated sensor a measuring pulse of the detected statevariable, said measuring pulse having a predetermined individual pulseduration, wherein in the second evaluating process the at least oneevaluating and control unit determines the individual pulse duration foreach encoding region. The encoder disc or the sensor is connected to thevehicle wheel. It is preferred that an encoder disc is in each casefixedly connected to a vehicle wheel and a corresponding sensor isarranged fixed to the vehicle body.

In an advantageous embodiment of the sensor arrangement in accordancewith the invention, in the second evaluating process the at least oneevaluating and control unit can calculate a mean pulse duration over awheel rotation as a quotient derived from the sum of the detectedindividual pulse durations of the encoding regions that are present onthe encoder disc and the number of the encoding regions that arepresent.

Generally, the encoder disc cannot be perfectly produced. As a result ofthe tolerances that occur, such as pitch error of the encoding regions,the measured pulse durations relating to a mean pulse duration that isdependent upon the rotational speed are not identical even in the caseof an ideal state, in other words in the case of a fixedly bolted,balanced wheel but are fixedly predetermined for each encoding region.As a result of wheel bolts becoming loose, a play occurs between thewheel and the wheel hub. As a result, small vibrations typically occurin the wheel suspension and also slippage occurs between the wheel andwheel hub in load changing situations. The two effects are evident inthe measured data of the sensors.

The vibration effect generates an additional periodic variation of theindividual pulse durations. The frequency of this variation correspondstypically to an integer multiple of the wheel rotational speed or ratherwheel frequency multiplied by the number of wheel bolts. In loadchanging situations, in other words during the transition from anaccelerating phase into a braking or rather motor braking phase or viceversa, slippage typically occurs between the wheel or rather the wheelrim and the wheel hub in the case of loosened wheel fastening means.This slippage is dependent upon the mechanical play between a wheelfastening means and its through-going opening in the wheel rim. In thecase of a given bolt hole radius, the theoretically possible slippageangle can be calculated in radians as a quotient. Typical values for theslippage angle lie in the range of 1 to 1.5 mm. The bolt hole radius hasa value of approximately 50 mm dependent upon the vehicle. Consequently,slippage angles in the range of approximately 1° are to be expected.

In a further advantageous embodiment of the sensor arrangement inaccordance with the invention, in the second evaluating process the atleast one evaluating and control unit can determine a periodic variationof the individual pulse durations over the rotational speed of thewheel. It is preferred that in the second evaluating process the atleast one evaluating and control unit performs a transformation of theperiodic variation into the frequency domain by means of filteringand/or Fourier transformation. The transition into the frequency domainrenders it possible in an advantageous manner to evaluate the frequencydependent variation of the individual pulse duration in a simple andrapid manner.

In a further advantageous embodiment of the sensor arrangement inaccordance with the invention, during the frequency analysis the atleast one evaluating and control unit determines amplitudes of spectrallines of the frequency spectrum of the periodic variation of theindividual pulse duration, the frequency of said spectral linescorresponding to an integer multiple of the wheel rotational speedmultiplied by the number of wheel fastening means. It is preferred thatthe evaluating and control unit compares the determined amplitudes ofthe spectral lines with predetermined threshold values and identifiesloosened wheel fastening means at the corresponding vehicle wheel if thedetermined amplitudes of the spectral lines achieve and/or exceed thepredetermined threshold value.

In a further advantageous embodiment of the sensor arrangement inaccordance with the invention, the evaluating and control unit candetermine a slippage angle of slippage that occurs in load changingsituations between the at least one vehicle wheel and the correspondingwheel hub as a short term sum from a difference of an instantaneouswheel rotational speed that can be calculated from the individual pulseduration, and a mean wheel rotational speed that can be determined fromthe mean pulse duration and/or from the short term sum by way of thevariation and can evaluate the mechanical play between the at least onevehicle wheel and the corresponding wheel hub. It is preferred that theat least one evaluating and control unit identifies loosened wheelfastening means at the corresponding vehicle wheel if the value of thecalculated slippage angle achieves a predetermined threshold value of byway of example 1° and/or is in a predetermined tolerance range of by wayof example ±0.2° around this threshold value.

In a further advantageous embodiment of the sensor arrangement inaccordance with the invention, the at least one evaluating and controlunit can indicate loosened wheel fastening means by means of an opticaland/or acoustic warning signal wheel fastening means.

An exemplary embodiment of the invention is illustrated in the drawingsand is further explained in the description hereinunder. In thedrawings, identical reference numerals describe components or ratherelements that perform the same or rather similar functions.

SHORT DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic block diagram of an exemplary embodimentof a sensor arrangement in accordance with the invention for a vehicle.

FIG. 2 illustrates a schematic perspective illustration of a rotationalspeed detecting device for the sensor arrangement in accordance with theinvention for a vehicle from FIG. 1.

FIG. 3 illustrates a schematic illustration of the connection betweenthe movement of a multipole disc and a measurement of the change in themagnetic field.

FIG. 4 illustrates a schematic plan view of a connecting region havingfive bolted connections between a wheel rim and a wheel hub so as toillustrate a mechanical slippage.

FIG. 5 illustrates a schematic cross sectional view of a boltedconnection between the wheel rim and the wheel hub, said boltedconnection being arranged in the connecting region in FIG. 8.

FIG. 6 illustrates a schematic illustration of a first pole regionspectrum that was determined using the sensor arrangement in accordancewith the invention from FIG. 1 at a vehicle wheel having five fixedwheel fastening means.

FIG. 7 illustrates a schematic illustration of a second pole regionspectrum that was determined using the sensor arrangement in accordancewith the invention from FIG. 1 at a vehicle wheel having five loosenedwheel fastening means.

FIG. 8 illustrates a schematic illustration of a third pole regionspectrum that was determined using the sensor arrangement in accordancewith the invention from FIG. 1 at a vehicle wheel having three fixedwheel fastening means.

FIG. 9 illustrates a schematic illustration of a fourth pole regionspectrum that was determined using the sensor arrangement in accordancewith the invention from FIG. 1 at a vehicle wheel having three loosenedwheel fastening means.

FIG. 10 illustrates a schematic characteristic curve diagram forillustrating an instantaneous wheel rotational speed and a mean wheelrotational speed that were determined using the sensor arrangement inaccordance with the invention from FIG. 1 at a vehicle wheel.

EMBODIMENTS OF THE INVENTION

As is evident from FIGS. 1 to 5, the illustrated exemplary embodiment ofa sensor arrangement in accordance with the invention for a vehicle 1comprises multiple rotational speed detecting devices 20, 22, 24, 26, 28that are embodied in the illustrated exemplary embodiment as magneticrotational speed detecting devices 20, 22, 24, 26, 28 and thatcontinuously detect at least one state variable S of a magnetic field Mand output said state variable to an evaluating and control unit 10 thatis arranged centrally within the vehicle 1. As is further evident inFIGS. 2 and 3, alternatively each rotational speed detecting device 20can be allocated an evaluating and control unit 100. The at least oneevaluating and control unit 10, 100 receives signals from at least onerotational speed detecting device 20, 22, 24, 26, 28 and evaluates saidsignals. In a first evaluating process 12, the at least one evaluatingand control unit 10, 100 determines a rotational speed ω of at least onevehicle wheel 30 whose wheel rim 32 is fastened by way of wheelfastening means 34 to a corresponding wheel hub 52. In accordance withthe invention, the at least one evaluating and control unit 10, 100performs a second evaluating process 14 and based upon the continuouslydetected at least one state variable S identifies and monitors amechanical play between the at least one vehicle wheel 30 and thecorresponding wheel hub 52 so as to detect loosened wheel fasteningmeans 34. The at least one evaluating and control unit 10, 100 indicatesloosened wheel fastening means 34 by means of an optical and/or acousticwarning signal that is output by way of outputting means, notillustrated, that comprise by way of example warning lamps, loudspeakers etc.

As is further evident in FIGS. 2 and 3, each rotational speed detectingdevice 20, 22, 24, 26, 28 comprises an encoder disc that is preferablyembodied as a magnetic encoder disc or rather multipole disc 21 having apredetermined number N of pole regions 21.1, and an allocated sensorthat is embodied as a magnetic field sensor 23. Each pole region 21.1generates in the allocated magnetic field sensor 23 a measuring pulse ofthe detected state variable S, said measuring pulse having apredetermined individual pulse duration Δt_(i), wherein in the secondevaluating process 14 the at least one evaluating and control unit 10,100 determines the individual pulse duration Δt_(i) for each pole region21.1, wherein the multipole disc 21 or the magnetic field sensor 23 isconnected to the vehicle wheel 30 that is mounted in a rotatable manner.In the illustrated exemplary embodiment, a multipole disc 21 is fixedlyconnected in each case to a vehicle wheel 30, and a correspondingmagnetic field sensor 23 is arranged fixed to the vehicle body. When anevaluating and control unit 100 is used in each case for each vehiclewheel 30, the evaluating and control units 100 form in each case with acorresponding magnetic field sensor 23 preferably a sensor unit 25 thatis arranged in a common housing 27.

The rotational speed detecting devices 20, 22, 24, 26, that are arrangedin the motor vehicle at each vehicle wheel 30 so as to use the ABSfunctionality measure the individual rotational speed of each vehiclewheel 30. For this purpose, the magnetic field sensor 23 that isattached to the vehicle, said magnetic field sensor being embodied byway of example as a Hall sensor or GMR sensor (Giant MagnetoResistance), measures a period of time that is necessary for themultipole disc that is arranged on the wheel to further rotate about apredetermined angular position. The measurement of the movement of themultipole disc 21 is performed by way of a measurement of the change inthe magnetic field M, whose field lines are schematically illustrated inFIG. 3. The signal curve that is illustrated in FIG. 3 for the detectedstate variable S of the magnetic field M occurs in dependence upon theposition of the multipole disc 21 in relation to the magnetic fieldsensor 23. A magnetic field strength B is preferably measured as a statevariable S. The predetermined angular position corresponds to thedimensions of the individual pole regions 21.1 and the period of timecorresponds to the above mentioned individual pulse duration Δt_(i).

Generally, the multipole disc 21 cannot be perfectly produced. As aresult of the tolerances that occur, pole region pitch errors and themeasured individual pulse durations Δt_(i) occur tooth durations relatedto a mean pulse duration Δt_(mean) that is dependent upon the rotationalspeed are not identical even in the case of an ideal state, in otherwords, fixedly bolted, balanced vehicle wheel 30. However, theindividual pulse duration Δt_(i) is fixed predetermined for each poleregion 21.1. A mechanical play occurs between the wheel rim 32 or ratherthe vehicle wheel 30 and the wheel hub 52 as a result of wheel fasteningmeans 34 becoming loose. As a result, small vibrations typically occurin the wheel suspension, and also slippage d_(S) occurs between thewheel rim 32 or rather the vehicle wheel 30 and the wheel hub 52 in loadchanging situations. The two effects are evident in the measurement dataof the magnetic field sensors 23.

The vibration effect generates an additional periodic variation ε_(i) ofthe individual pulse duration Δt_(i). The frequency of this variationε_(i) typically corresponds to an integer multiple of the rotationalspeed ω of the corresponding vehicle wheel 30 that is multiplied by thenumber N of wheel fastening means 34. The at least one evaluating andcontrol unit 10, 100 determines the individual pulse duration Δt_(i) byway of example by way of the points in time of the zero crossings of thestate variable S.

FIGS. 6 to 9 illustrate different frequency spectrums of this variationε_(i). The frequency lines in accordance with a discrete Fouriertransformation of the variation ε over an entire wheel rotation areillustrated. For this purpose, in the second evaluating process the atleast one evaluating and control unit 10, 100 forms for each pole region21.1 the individual periodic variation ε_(i) as a wheel rotationalspeed-dependent ratio of an individual, measured pulse duration Δt_(i)and a mean pulse duration Δt_(mean) minus a constant having the value 1in accordance with equation (1).

$\begin{matrix}{ɛ_{i} = {\frac{\Delta \; t_{i}}{\Delta \; t_{mean}} - 1}} & (1)\end{matrix}$

In the second evaluating process 14, the at least one evaluating andcontrol unit 10, 100 calculates the mean pulse duration Δt_(Mean) over awheel rotation as a quotient derived from the sum of the detectedindividual pulse durations Δt_(i) of the pole regions 21.1 that areprovided on the multipole disc 21 and the number N of the provided poleregions 21.1 in accordance with equation (2).

$\begin{matrix}{{\Delta \; t_{Mean}} = {\frac{1}{N}{\sum\limits_{k = 0}^{N - 1}{\Delta \; t_{i - k}}}}} & (2)\end{matrix}$

As an alternative to the Fourier transformation, the at least oneevaluating and control unit 10, 100 can transform the periodic variationε_(i) in the second evaluating process 14 by means of filtering into thefrequency domain.

FIG. 6 illustrates in an exemplary manner a first pole region spectrumthat was determined using the sensor arrangement in accordance with theinvention at a vehicle wheel 30 having five fixed wheel fastening means34. FIG. 7 illustrates a second pole region spectrum that is measured atthe same vehicle wheel 30 in the case of loosened wheel fastening means34. As is evident from a comparison of FIGS. 6 and 7, spectral lines ofthe second pole region spectrum in accordance with FIG. 7 at frequenciesthat correspond to an integer multiple of the wheel frequency f_(Wheel)multiplied by the number N of wheel fastening means 34 comprise aclearly higher amplitude than the corresponding spectral lines of thefirst pole region spectrum in accordance with FIG. 6. This applies inparticular for 5 times or rather 10 times etc. the wheel frequencyf_(Wheel).

FIG. 8 illustrates in an exemplary manner a third pole region spectrumthat was determined using the sensor arrangement in accordance with theinvention at a vehicle wheel 30 having three fixed wheel fastening means34. FIG. 9 illustrates a fourth pole region spectrum that is measured atthe same vehicle wheel 30 in the case of loosened wheel fastening means34. As is evident from comparing FIG. 8 and FIG. 9, spectral lines ofthe fourth pole region spectrum in accordance with FIG. 9 at frequenciesthat correspond to an integer multiple of the wheel frequency f_(Wheel)multiplied by the number N of wheel fastening means 34 comprise aclearly higher amplitude than the corresponding spectral lines of thethird pole region spectrum in accordance with FIG. 8. This applies inparticular for 3 times or rather 6 times or rather 9 times etc. thewheel frequency f_(Wheel).

The at least one evaluating and control unit 10, 100 determines theamplitudes of the spectral lines of the frequency spectrum of theperiodic variation ε_(i) of the individual pulse duration Δt_(i), thefrequency of said spectral lines corresponding to an integer multiple ofthe wheel rotational speed ω or rather wheel frequency f_(Wheel)multiplied by the number N of wheel fastening means 34. The at least oneevaluating and control unit 10, 100 during the frequency analysiscompares the determined amplitudes of the spectral lines with thepredetermined threshold values and identifies loosened wheel fasteningmeans 34 at the corresponding vehicle wheel 30 if the determinedamplitudes of the spectral lines achieve and/or exceed the predeterminedthreshold values. Alternatively, detection can only occur at increasedvariation ε_(i) of the pole region pitch.

In addition or alternatively, the at least one evaluating and controlunit 10, 100 uses the slippage effect and evaluates slippage that isdetermined from the wheel rotational speed signals so as to detectloosened wheel fastening means 34.

As is further evident in FIGS. 4 and 5, in load changing situations, inother words during the transition from an accelerating phase into abraking phase or rather motor braking phase or vice versa, slippagetypically occurs between the wheel rim 32 and the wheel hub 52 in thecase of loosened wheel fastening means 34. This slippage is dependentupon mechanical play d_(s) between a corresponding wheel fastening means34 and its through-going opening in the wheel rim 32. In the case of agiven bolt hole radius r_(BH), the theoretically possible slippage angleφ_(s) is calculated in radians as a quotient in accordance with equation(3).

$\begin{matrix}{\phi_{S} = \frac{d_{S}}{r_{BH}}} & (3)\end{matrix}$

As is further evident in FIG. 4, the wheel bolts 34 are engaged at onehole side. In the case of a change in load, a change occurs to the otherhole side that is illustrated by the dashed line. Typical values for themechanical play d_(s) lie in the region of 1 to 1.5 mm. The bolt holeradius r_(BH) is dependent upon the vehicle and has a value ofapproximately 50 mm. Slippage angles φ_(S) in the magnitude ofapproximately 1° are to be expected.

As is evident in FIG. 10, the at least one evaluating and control unit10, 100 in the illustrated exemplary embodiment determines the slippageangle φ_(S) from the measured wheel rotational speed signals ω, whereinthe at least one evaluating and control unit 10, 100 calculates a shortterm integer φ_(S) _(_) _(Measured) of a difference of instantaneouswheel rotational speed ω_(i) that is calculated from the individualpulse duration Δt_(i), and the mean rotational speed ω_(Mean) that iscalculated from the mean pulse duration (Δt_(Mean)) in accordance withequation (4).

φ_(S) _(_) _(Measured)=∫(ω_(i)−ω_(Mean))dt  (4)

This corresponds approximately to the short term sum φ_(S) _(_)_(Measured) in accordance with equation (5).

$\begin{matrix}{\phi_{S\_ Measured} \approx {\sum{{\left( {\frac{2\pi}{{N \cdot \Delta}\; t_{i}} - \frac{2\pi}{{N \cdot \Delta}\; t_{Mean}}} \right) \cdot \Delta}\; t_{i}}}} & (5)\end{matrix}$

or rather in accordance with the transformation of the weighted shortterm sum φ_(S) _(_) _(Measured) over the variation ε_(i) in accordancewith equation (6).

$\begin{matrix}{{\phi_{S\_ Measured}}_{,i} = {{- \frac{2\pi}{N}}{\sum\limits_{k = 0}^{4}{ɛ_{i - k}.}}}} & (6)\end{matrix}$

The calculated slippage angle φ_(S) is signed, wherein the signdiscloses whether the instantaneous wheel rotational speed ω_(i) has avalue below or above the averaged wheel rotational speed ω_(Mean).During a transition from an accelerating phase into a braking phase, alower instantaneous rotational speed ω_(i) is to be expected and in thecase of a load change in the opposite direction, a higher instantaneouswheel rotational speed ω_(i) is to be expected.

It is therefore preferred that the weighted short term sum φ_(S) _(_)_(Measured,i) that is continuously disclosed in equation (6) is formedover the variation ε_(i) so as to detect the slippage effect, by way ofexample over five neighboring pole regions 21.1. This can be implementedby way of example as an FIR-Filter (FIR: Finite Impulse Response) havingthe length 5, wherein all the filter coefficients comprise the value 1.The summation length of 5 comes from the expectation that the changefrom one side to another is typically completed within a rollingdistance that corresponds to five pole regions 21.1 on the multipoledisc 21 or rather approximately to a tenth of a wheel rotation. Theevaluating and control unit 10, 100 evaluates the calculated slippageangle φ_(S) of the slippage that occurs so as to identify the mechanicalplay d_(s) between the at least one vehicle wheel 30 or rather wheel rim32 and the corresponding wheel hub 52 in load changing situations. Theat least one evaluating and control unit 10, 100 identifies loosenedwheel fastening means 34 at the corresponding vehicle wheel 30 if thevalue of the calculated slippage angle φ_(S) achieves a predeterminedthreshold value and/or is in a predetermined tolerance range around thisthreshold value. The repeated occurrence of measured slippage anglesφ_(S) _(_) _(Measured,i) that deviate slightly from the theoreticalvalue φ_(S) during the load changing situation can likewise be used soas to detect loosened wheel fastening means 34.

Embodiments of the sensor arrangement in accordance with the inventionfor a vehicle 1 can be implemented in an advantageous manner withoutadditional outlay on hardware in the ESP control device or ABS controldevice of each vehicle. It is possible in this manner for embodiments ofthe sensor arrangement in accordance with the invention to be usedpotentially in any vehicle such as for example passenger cars, heavygoods vehicles, motorbikes that comprise wheel rotational speed sensorsat the wheels that are to be monitored.

The invention has been described using an example of magnetic rotationalspeed detecting devices that comprise multipole discs as encoder discsand magnetic field sensors so as to detect at least one state variableof a magnetic field. Clearly, it is also possible to use embodiments ofthe present invention also having rotational speed detecting devicesthat evaluate other physical variables, such as by way of exampleoptical variables, so as to detect the rotational speed.

1. A sensor arrangement for a vehicle, the sensor arrangementcomprising: at least one evaluating and control unit; and at least onerotational speed detecting device configured to (i) continuously detectat least one state variable and (ii) output to the at least oneevaluating and control unit, wherein the at least one evaluating andcontrol unit is configured to (i) perform a first evaluating processusing the detected at least one state variable, (ii) determine arotational speed of at least one wheel of the vehicle, the at least onewheel having a wheel rim fastened to a corresponding wheel hub by way ofwheel fastening means, (iii) perform a second evaluating process, and(iv) based upon the continuously detected at least one state variable,identify and monitor a mechanical play between the at least one wheeland the corresponding wheel hub so as to detect loosened wheel fasteningmeans.
 2. The sensor arrangement as claimed in claim 1 wherein: the atleast one rotational speed detecting device comprises an encoder dischaving a predetermined number of encoding regions and an allocatedsensor, each encoding region being configured to generate in theallocated sensor a measuring pulse of the detected at least one statevariable having an individual pulse duration; and the at least oneevaluating and control unit is configured to, in the second evaluatingprocess, determine the individual pulse duration for each encodingregion.
 3. The sensor arrangement as claimed in claim 2, wherein the atleast one evaluating and control unit is configured to, in the secondevaluating process, calculate a mean pulse duration over a wheelrotation as a quotient derived from a sum of the determined individualpulse durations of the encoding regions that are provided on the encoderdisc and the predetermined number of the provided encoding regions. 4.The sensor arrangement as claimed in claim 3, wherein the at least oneevaluating and control unit is configured to, in the second evaluatingprocess, determine a periodic variation of the individual pulsedurations over the wheel rotation.
 5. The sensor arrangement as claimedin claim 4, wherein the at least one evaluating and control unit isconfigured to, in the second evaluating process, perform atransformation of the periodic variation into a frequency domain usingat least one of filtering and Fourier transformation.
 6. The sensorarrangement as claimed in claim 4, wherein the at least one evaluatingand control unit is configured to determine amplitudes of spectral linesof a frequency spectrum of the periodic variation of the individualpulse duration, a frequency of said spectral lines corresponding to aninteger multiple of a wheel rotational speed multiplied by a number ofwheel fastening means.
 7. The sensor arrangement as claimed in claim 6,wherein the evaluating and control unit is configured to (i) compare thedetermined amplitudes of the spectral lines with predetermined thresholdvalues and (ii) identify loosened wheel fastening means in response tothe determined amplitudes at least one of equaling and exceeding thepredetermined threshold values.
 8. The sensor arrangement as claimed inclaim 4, wherein the evaluating and control unit is configured to (i)determine a slippage angle of slippage that occurs in load changingsituations between the at least one wheel and the corresponding wheelhub as a short term sum based on a difference between an instantaneouswheel rotational speed and a mean wheel rotational speed, theinstantaneous wheel rotational speed being calculated from theindividual pulse duration, the mean wheel rotational speed beingcalculated from at least one of the mean pulse duration and the shortterm sum by way of the periodic variation and (ii) evaluate saidslippage angle so as to identify the mechanical play between the atleast one vehicle wheel and the corresponding wheel hub.
 9. The sensorarrangement as claimed in claim 8, wherein the at least one evaluatingand control unit is configured to identify loosened wheel fasteningmeans of the at least one wheel in response to the value of thedetermined slippage angle is at least one of equal to a predeterminedthreshold value and within a predetermined tolerance range around thepredetermined threshold value.
 10. The sensor arrangement as claimed inclaim 1, wherein the rotational speed detecting device is embodied as atleast one of a magnetic rotational speed detecting device and an opticalrotational speed detecting device.
 11. The sensor arrangement as claimedin claim 1, wherein the at least one evaluating and control unit isconfigured to indicate loosened wheel fastening means using at least oneof an optical warning signal and an acoustic warning signal.